Slysh (1966) and
Mitton (1971)
first pointed out the `anomalous'
behavior of the polarized emission from Cygnus A, including changing
fractional polarization with observed wavelength (at low resolution),
essentially random projected electric field vectors, and a large
difference between the magnitude of Faraday rotation measure (RM)
towards the two lobes. This problem was delineated in detail in the
study of the rotation measure distribution towards Cygnus A by
Alexander et al. (1984).
They find that the `rotation measure values are very
noisy, and no overall pattern can be discerned.' They suggest a Galactic
origin for the rotation measures, based simply on the fact that Cygnus A
has a fairly low Galactic latitude (b = 5.8°).

The question of the large anomalous Faraday rotation towards Cygnus A
was resolved by the sensitive, multi-frequency, spatially resolving
polarization observations of
Dreher et al. (1987b).
These authors find
that Cygnus A lies behind a deep `Faraday screen', with rotation
measures varying from -4000 rad m-2 to +3000 rad m-2
across the source. Gradients in RM exceed 300 rad m-2
arcsec-1. In general the distribution is not random, but displays
structure with coherence over spatial scales of order 10 kpc. Perhaps
most importantly,
Dreher et al. (1987b)
find that the total rotation of
the position angle of the polarization vector exceeds 600° in
many regions, without departure from a
2 dependence, where
= observed wavelength, and
without depolarization
demonstrating conclusively that the Faraday rotation cannot be internal
to the source but must be by an external screen
(Burn 1966).
This conclusion has been further strengthened by the recent 8 GHz results of
Perley and Carilli
(1996).
The 8 GHz data set severe limits on deviations from a
2 dependence of
position angle throughout the observed wavelength range.

Dreher et al. (1987b)
consider, and reject, a Galactic origin for the
Faraday screen. They propose that the origin of the large RM's is in
the hot intracluster gas in which Cygnus A is embedded. The implication
is that the large-scale cluster gas is substantially magnetized. Using
the cluster gas density radial profile derived from X-ray observations,
Dreher et al. calculate cluster magnetic field strengths between 2
µG and 10 µG, depending on geometry. Also, from
the lack of
wavelength dependent fractional polarization at fixed resolution, Dreher
et al. derive an upper limit to the thermal electron density in the
radio lobes of 2 x 10-4 cm-3. This density limit depends
on the minimum energy assumption for the radio source fields and assumes
a uni-directional field through the lobes. Any field reversals along
the line of sight would lead to a less stringent electron density limit.

An alternative model for the large rotation measures towards Cygnus A
has been proposed by
Bicknell et al. (1990),
in which a thin
mixing-layer exists along the contact discontinuity, where shocked
cluster gas mixes with the large fields in the radio source. Support
for this idea comes from the `striated' appearance of the RM
distribution across the radio lobes, suggestive of large-scale K-H
instabilities at the contact discontinuity. However, since the
discovery of extreme rotation measures towards Cygnus A spatially
resolving observations of many radio galaxies at the centers of dense,
X-ray emitting cluster atmospheres have revealed large rotation measures
(for a summary, see
Taylor et al. 1994).
Taylor et al. show a
clear correlation between the cluster core thermal density and the
magnitude of RM's observed to cluster center radio sources. This
includes both luminous edge-brightened (FRII) sources, such as 3C 295
(Perley and Taylor 1991),
and less luminous edge-darkened (FRI) sources,
such as M 87
(Owen 1989).
iven that the physical interaction between
the source and its environment is thought to be very different for the
two types of sources
(DeYoung 1993),
we conclude that the majority of
the large RM's observed must be the result of substantially magnetized,
large scale cluster gas, and not simply the result of the interaction
between the source with its environments.

The discovery of the bow shock in the RM distribution in the northern
hotspot region in Cygnus A has relevance to this debate.
Carilli et al. (1988)
show that the observed RM change at the bow shock implies
pre-shock intracluster fields of 8 µG. They propose a simple model
in which the large-scale RM distribution towards Cygnus A (amplitudes up
to 4000 rad m-2, typical scale-sizes
10 kpc) is caused by
the unperturbed cluster atmosphere, while small scale fluctuations
(amplitude 1000 rad
m-2, scale-sizes 5 kpc)
can result from the interaction of the source and the ambient medium.

In summary, the detection of extreme rotation measures towards Cygnus A,
and subsequently in other cluster-center radio galaxies, shows that the
thermal cluster atmospheres must be substantially magnetized, with
fields a few µG. In
most cases the pressure in the
intracluster fields is below the thermal energy density (e.g. the
plasma -factor =
ratio of thermal to magnetic pressure is between
20 and 50 for the Cygnus A cluster), implying a minor dynamical role for
the fields. Even dynamically unimportant fields alter substantially the
thermal conductivity of the plasma, and hence are very important when
considering the cooling and heating of the ICM
(Sarazin 1986,
1988).
Lastly, the detailed study of the Faraday screen towards Cygnus A
provides vital supporting evidence for physical models explaining the
depolarization asymmetries towards extragalactic radio sources and their
implication for quasar-powerful radio galaxy unification schemes
(Laing 1988,
Garrington et al. 1988,
1991).

Models for the origin of intracluster fields include: the dynamo action
of turbulent wakes of galaxies, injection of fields by previous
outbursts of the radio source, ram pressure stripping of fields from
galaxies, and amplification of a primordial field
(Jaffe 1980,
Ruzmaikin et al. 1989,
Jafelice and Opher
1992; see
Sarazin 1986,
1988 for
reviews). Another possible mechanism for generating large scale cluster
fields is amplification of seed fields during the merger of two clusters.
Eilek (1995)
presents a detailed `Zeldovich rope dynamo'
model for field generation in a cluster with net-helical turbulence,
presumably driven by galaxy motions. She finds that the fields
eventually evolve to equipartition strengths. She predicts RM
distributions towards cluster center sources which compare well with
observations, both in amplitude and structure.